It is already known (DE 26 46 172 C2) to rearrange ketoisophorone in the gas phase with a zeolite to yield the trimethylhydroquinone. However, the yields of this reaction are only low (50% at 30% conversion) and are thus unsatisfactory for an economic process. In another process (Y. A. Joe, Y. M. Goo, Y. Y. Lee, Bull. Korean Chem. Soc. 1991, 12, 253), the rearrangement is performed in a 5% solution in acetic anhydride by the addition of five equivalents of concentrated sulfuric acid. Trimethylhydroquinone esters are obtained in this process at a yield of only 31%, such that this process is also uneconomic. According to a third method (DE-OS 2 149 159), ketoisophorone may be reacted in acetic anhydride in the presence of a protonic acid to yield trimethylhydroquinone diacetate, which is subsequently saponified to yield 2,3,5-trimethylhydroquinone. Disadvantages of this process include the use of
large quantities of acetic anhydride (5-10 mol./mol. of ketoisophorone), PA1 large quantities of the catalytic acid (up to 150 mol. %) PA1 together with only moderate yields of at most 66%. PA1 Yields from the process according to the invention are up to approx. 25% higher than in the cited literature and are between 85 and 95%. PA1 The required quantities of catalyst are 0.1 to 50%, in comparison with up to 150% in the literature. PA1 Only &gt;2 to 4 mol. of a carboxylic anhydride are required per mol. of ketoisophorone, in comparison with 5 to 10 mol. in the literature.